I really am sorry — being overweight is neither fun nor pleasant nor easy on the mind — but in looking up actual airline policies, all of them permit the overweight passenger to purchase a second seat.
It seems to me that your distress, while beautifully written, stems in large part from not wanting to spend the money to do that. I get that there are cost constraints, and that a second seat is not cheap.
But, I strongly believe that it is entirely up to you to make the relevant accommodations. If you suspect, much less know, that you will not comfortably fit into a seat, you are welcome to purchase a second seat and to discuss the matter with the airline (with or without lawyers). That is in your hands.
I do not believe it is up to the passengers beside you to deal with your weight. I agree that they shouldn’t judge you based on your weight, but in this instance, your weight is making not only your flight apparently miserable but also theirs. So, in that regard, I consider this partially your fault and your responsibility.
Yes, we should be kind and civil and understanding of one another. We should not simply get angry at an overweight passenger, but knowing that the passenger in question chose to cram themselves into this seat kind of takes some of that civility down a notch.
Just my $.02 after spending several flights with overweight passengers next to me. It’s was not a pleasant experience on any of those occasions.
Edited: I was also thinking strongly that being overweight might constitute a disability. Again, making arrangements in that case should be between you and the airline. And it should generally involve a scenario that is reasonable for you. It sounds like a single seat is not one such scenario.